Beam intensity modulation for penumbra enhancement and field length reduction in lung cancer treatments: a dosimetric study.

Abstract

In a recent treatment planning study, a previously published technique for superior-inferior field length reduction for prostate cancer patients, based on penumbra enhancement using static beam intensity modulation (BIM) with a multileaf collimator, was investigated for lung cancer treatments. For the patient group studied, the field lengths could be reduced by 1.4 cm and an average dose escalation of 6 Gy (maximum 16 Gy) appeared to be possible without any increase in the calculated risk of radiation pneumonitis. However, this planning study was performed with a treatment planning system that does not correctly account for the increased lateral secondary electron transport in lung tissue, resulting in too steep beam penumbrae. Therefore, prior to clinical implementation, an extensive dosimetric study was performed to evaluate and optimize BIM for penumbra enhancement and superior-inferior field length reduction in lung cancer treatments. Film dosimetry was performed in several phantoms consisting of water equivalent and lung equivalent materials, both for a 6 and a 10 MV photon beam. Measured dose distributions were used to (i) adapt the BIM technique to properly account for increased lateral secondary electron transport, (ii) compare BIM dose distributions in lung material with dose distributions of standard treatment fields, and (iii) investigate the use of our treatment planning system for the design of BIM plans for lung cancer patients. Compared with our treatment planning study the superior and inferior boost fields, used in the BIM technique for penumbra enhancement, had to be longer and of a higher weight to compensate for the increased lateral secondary electron transport in lung tissue. With these modifications in the BIM technique, field lengths could indeed be reduced by 1.4 cm compared with treatment with standard fields, without the appearance of underdosages in the most superior and inferior target areas, whilst better sparing the healthy lung tissue. Practical rules were derived to use our treatment planning system for the design of BIM treatment plans. In spite of the increased lateral secondary electron transport in lung tissue, static BIM with a multileaf collimator may effectively be used for penumbra enhancement and superior-inferior field length reduction in lung cancer treatments.